FOR PARTICULATE FILTER AND PARTICULATE FILTER

REGENERATING METHOD

BACKGROUND OF THE INVENTION

1. Field of the Invention [0001] The invention relates to a method for estimating the amount of particulate matter remaining on a particulate filter, which is provided in an exhaust passage of an internal combustion engine and traps particulate matter contained in exhaust gas, during filter regeneration control which oxidizes and removes particulate matter that has accumulated on the particulate filter. The invention also relates to a method for regenerating a particulate filter.

2. Description of the Related Art [0002] In an internal combustion engine provided with a particulate filter (hereinafter simply referred to as "filter") that traps particulate matter (hereinafter simply referred to as "PM") in exhaust gas in an exhaust passage, filter regeneration control is performed which oxidizes and removes PM that has accumulated on the filter by raising the temperature of the filter. [0003] Further, JP(A) 2003-293733 discloses technology which estimates the amount of PM remaining on the filter during execution of filter regeneration control (hereinafter also referred to as "PM remaining amount") based on the amount of PM accumulated on the filter at the start of execution of the filter regeneration control (hereinafter also referred to as "PM accumulation amount"). Here, the amount of PM removed per unit time during the execution of filter regeneration control (hereinafter simply referred to as "PM removal amount") is determined according to the PM accumulation amount at the start of filter regeneration control. The amount of change from the amount of PM accumulated on the filter at the start of filter regeneration control is calculated by adding up the amount of PM removed per unit time. The PM remaining amount accumulated on the filter during filter regeneration control is estimated from the calculated amount of change and the PM accumulation amount at the start of filter regeneration control. Also, the PM removal amount per unit time during the execution of filter regeneration control is set to be larger the greater the PM accumulation amount at the start of execution of the filter regeneration control. [0004] Also, Patent No. 2616074 and JP(A) 2000-170521 are examples of documents relating to this invention. [0005] In filter regeneration control, in order to make the end timing of that control more appropriate, it is necessary to know the PM remaining amount while the filter regeneration control is being executed. Further, when the filter regeneration control is stopped prematurely, i.e., when the filter regeneration control is stopped while there is still PM on the filter, and the PM remaining amount at the time that the filter regeneration control was stopped is not known, it also becomes difficult to know the PM accumulation amount after that point. As a result, the next cycle of filter regeneration control might not be able to be started at the appropriate timing.

SUMMARY OF THE INVENTION [0006] In view of the foregoing problems, this invention thus provides technology that enables the amount of PM remaining on the filter during execution of filter regeneration control to be estimated with greater accuracy. [0007] Thus, one exemplary embodiment of the invention relates to a method for estimating the amount of PM remaining on a filter, which is provided in an exhaust passage of an internal combustion engine and traps PM in exhaust gas, during execution of filter regeneration control which oxidizes and removes PM that has accumulated on the filter, by raising the temperature of the filter. This filter PM remaining amount estimating method estimates the PM remaining amount based on a PM removal rate per unit time during execution of the filter regeneration control, a PM accumulation amount at the start of execution of the filter regeneration control, and the time elapsed from the start of execution of the filter regeneration control. [0008] When PM that has accumulated on the filter is removed by filter regeneration control, the PM removal amount per unit time changes according to the PM accumulation amount. That is, during execution of the filter regeneration control, the PM accumulation amount gradually reduces over time. As a result, the PM removal amount per unit time also gradually decreases. [0009] Therefore, when the PM removal amount during execution of the filter regeneration control is calculated by determining the PM removal amount per unit time according to the PM accumulation amount at the start of regeneration and then adding up the PM removal amounts per unit time, that calculated PM removal amount may be a larger value than the actual PM removal amount. Therefore, when the PM remaining amount during execution of the filter regeneration control is calculated by subtracting the thus calculated PM removal amount from the PM accumulation amount at the start of execution of the filter regeneration control, that calculated PM remaining amount may be off from the actual PM remaining amount. [0010] Therefore, this invention uses the PM removal rate per unit time during execution of the filter regeneration control to estimate the PM remaining amount during execution of the filter regeneration control. The PM removal rate in this case is the PM removal amount per unit PM accumulation amount. That is, the PM removal rate per unit time is the PM removal amount per unit PM accumulation amount in a unit of time, i.e., (PM removal amount per unit PM accumulation amount) per unit time. This PM removal rate per unit time can be determined beforehand through experimentation or the like. [0011] The PM removal rate per unit time does not change even if the PM accumulation amount gradually decreases over time during execution of the filter regeneration control. Therefore, the invention makes it possible to more accurately estimate the PM remaining amount during execution of the filter regeneration control. [0012] In this invention, the PM remaining amount may be estimated every time a specified period of time passes, by starting to count the elapsed time at the start of execution of the filter regeneration control, and, every time a specified period of time in this elapsed time is counted, subtracting the product of the specified period of time, the PM removal rate per unit time, and the PM remaining amount at the point when counting of the specified period of time started from the PM remaining amount at the point when counting of the specified time started. [0013] In this case, the specified period of time is preferably a preset value and as short as possible. For example, the specified period of time may be a unit of time which determines the PM removal rate per unit time. [0014] According to the method described above, the PM accumulation amount at the point when the specified period of time has been counted from the start of execution of the filter regeneration control, i.e., the PM accumulation amount at the point when the specified period of time has passed from the start of execution of the filter regeneration control, is calculated by subtracting the product of the specified period of time, the PM removal rate per unit time, and the PM accumulation amount at the start of the filter regeneration control from the PM accumulation amount at the start of filter regeneration control. [0015] Then, the PM accumulation amount (PM remaining amount) gradually decreases as time passes after the start of execution of the filter regeneration control. However, after the specified period of time has passed from the start of execution of the filter regeneration control, and every time the specified period of time is counted, i.e., every time the specified period of time passes, that specified period of time, the PM removal rate per unit time, and the PM remaining amount at the point at which the specified period of time at that time started to be counted are multiplied together to calculate the PM removal amount to be removed from the filter during the specified period of time at that time. This PM removal amount is then subtracted from the PM remaining amount at the point at which the specified period of time at that time started to be counted to calculate the PM remaining amount at the point at which the specified period of time at that time was counted. [0016] This kind of method enables the PM removal amount to be removed every time the specified period of time passes to be calculated more accurately during execution of the filter regeneration control. As a result, the PM remaining amount every time the specified period of time passes can be estimated with greater accuracy. [0017] In the foregoing method for estimating the amount of particulate matter remaining on a filter, the PM removal rate per unit time may be set to a higher value the greater the flow rate of exhaust gas flowing into the filter. [0018] This is because the heat quantity and oxygen amount supplied to the PM accumulated on the filter increases the greater the flow rate of the exhaust gas flowing into the filter, which tends to promote oxidation and removal of PM by the filter regeneration control. [0019] As a result, the PM removal rate per unit time can be set to a more accurate value, thus enabling the PM remaining amount during execution of filter regeneration control to be estimated more accurately. [0020] In the foregoing method for estimating the amount of particulate matter remaining on a filter, the PM removal rate per unit time may be set to a higher value the greater the oxygen concentration in the exhaust gas flowing into the filter. [0021] This is because the oxygen amount supplied to the PM accumulated on the filter increases the greater the oxygen concentration in the exhaust gas flowing into the filter, which tends to promote oxidation and removal of PM by the filter regeneration control. [0022] As a result, the PM removal rate per unit time can be set to a more accurate value, thus enabling the PM remaining amount during execution of filter regeneration control to be estimated more accurately. [0023] In the foregoing method for estimating the amount of particulate matter remaining on a filter, the PM removal rate per unit time may be set to a higher value the higher the temperature of the filter. [0024] This is because higher filter temperatures tend to promote the oxidation and removal of PM by the filter regeneration control. [0025] As a result, the PM removal rate per unit time can be set to a more accurate value, thus enabling the PM remaining amount during execution of filter regeneration control to be estimated more accurately. [0026] Another exemplary embodiment of the invention relates to a method for regenerating a filter. When the operating state of the internal combustion engine changes during execution of the filter regeneration control, the regenerating method calculates the PM remaining amount at the time of that change according to the method for estimating the amount of PM remaining on a filter described above which changes the PM removal rate per unit time according to any one of at least a flow rate of exhaust gas flowing into the filter, an oxygen concentration in the exhaust gas flowing into the filter, and the temperature of the filter. A duration of the filter regeneration control necessary to oxidize and remove the remaining PM is then calculated based on the PM remaining amount at that time and the PM removal rate per unit time which is set according to the new (i.e., changed) operating state of the internal combustion engine. The filter regeneration control stops when that duration is equal to, or greater than, a specified duration. [0027] When the operating state of the internal combustion engine changes, the flow rate, oxygen concentration, and temperature of the exhaust gas flowing into the filter may also change. When those change, the temperature of the filter may also change. Therefore, when the operating state of the internal combustion engine changes during execution of the filter regeneration control, the PM removal rate per unit time may be different after the change than it was before the change. [0028] If the PM removal rate per unit time drops due to a change in operating state of the internal combustion engine, the duration of the filter regeneration control necessary to oxidize and remove the remaining PM becomes longer. If the filter regeneration control is executed for too long, however, it may lead to deterioration in exhaust gas emissions. [0029] Thus, the invention calculates the duration of the filter regeneration control necessary for oxidizing and removing the remaining PM based on the PM remaining amount at the point when the operating state of the internal combustion engine changed and the PM removal rate per unit time that is set according to the operating state of the internal combustion engine after the operating state has changed. The filter regeneration control is then stopped when the calculated duration is equal to, or greater than, a specified duration. [0030] The specified duration in this case is a period of time that is equal to, or less than, a threshold value at which it can be determined that there is a risk of leading to a deterioration in fuel efficiency or exhaust gas emissions due to the filter regeneration control being executed for too long. [0031] The invention makes it possible to inhibit the filter regeneration control from being executed for too long. As a result, it is possible to suppress deterioration in fuel efficiency and exhaust gas emissions. [0032] The method for estimating the amount of particulate matter remaining on a particulate filter according to the invention makes it possible to more accurately estimate the PM remaining amount during execution of filter regeneration control.

BRIEF DESCRIPTION OF THE DRAWINGS [0033] The above mentioned and other features, advantages, technical and industrial significance of this invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: FIG. 1 is a schematic diagram of an internal combustion engine and intake and exhaust systems thereof according to first and second exemplary embodiments of the invention; FIG. 2 is a graph showing the shift in PM remaining amount during execution of filter regeneration control; FIG. 3 is a flowchart illustrating a PM remaining amount estimating routine for estimating the PM remaining amount during execution of filter regeneration control, according to the first and second exemplary embodiments of the invention; and FIG. 4 is a flowchart illustrating a routine for controlling the stopping or continuation of the filter regeneration control, according to the second exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0034] In the following description and the accompanying drawings, the present invention will be described in more detail in terms of exemplary embodiments. A first exemplary embodiment of the invention will first be described. FIG. 1 is a schematic diagram of an internal combustion engine and intake and exhaust systems thereof according to this exemplary embodiment. The internal combustion engine 1 is a diesel engine for driving a vehicle. A piston 3 is slidably provided in a cylinder 2 of the internal combustion engine 1. An intake port 4 and an exhaust port 5 are connected to a combustion chamber in an upper portion in the cylinder 2. The portion of the intake port 4 which is open to the combustion chamber is opened and closed by an intake valve 6, and the portion of the exhaust port 5 which is open to the combustion chamber is opened and closed by an exhaust valve 7. The intake port 4 is connected to an intake passage 8 and the exhaust port 5 is connected to an exhaust passage 9. Also, a fuel injection valve 10 which injects fuel directly into the cylinder 2 is provided in the cylinder 2. [0035] The intake passage 8 is provided with a throttle valve 15 which controls the intake air amount and an airflow meter 16 which outputs an electric signal indicative of the intake air amount. [0036] The exhaust passage 9 is provided with a filter 11 that traps PM in the exhaust gas. This filter 11 carries an oxidation catalyst. The catalyst carried on the filter 11 need only have an oxidizing function, so a NOx storage-reduction catalyst, for example, may be used. Also, the catalyst does not have to be carried on the filter 11 itself, i.e., the catalyst may be disposed in the exhaust passage 9 upstream of the filter 11. [0037] A fuel adding valve 12 which adds fuel to the exhaust gas is provided in the exhaust passage 9 upstream of the filter 11. An oxygen concentration sensor 14 that outputs an electric signal indicative of the oxygen concentration in the exhaust gas is provided in the exhaust passage 9 upstream of the filter 11 and downstream of the fuel adding valve 12. Also, an exhaust gas temperature sensor 13 which outputs an electric signal indicative of the temperature of the exhaust is provided in the exhaust passage 9 downstream of the filter 11. [0038] An ECU 20 for controlling the internal combustion engine 1 is disposed adjacent to the internal combustion engine 1 which is structured as described above. Various sensors are connected to the ECU 20 via electrical wiring, through which they send output signals to the ECU 20. Some of these sensors include the exhaust gas temperature sensor 13, the oxygen concentration sensor 14, the airflow meter 16, a crank position sensor 17 which outputs an electric signal indicative of the crank angle, and an accelerator opening amount sensor 18 which outputs an electric signal indicative of the accelerator opening amount. The ECU 20 is also electrically connected to the fuel injection valve 10, the fuel adding valve 12, and the throttle valve 15, which are controlled by the ECU 20. [0039] Next, the filter regeneration control will be described. In this exemplary embodiment, the filter regeneration control for oxidizing and removing accumulated PM starts when the PM accumulation amount at the filter 11, which is estimated based on the accumulated fuel injection quantity from the fuel injection valve 10 and the like, becomes a preset PM accumulation amount Qst for starting regeneration. In the filter regeneration control according to this exemplary embodiment, the temperature of the filter 11 rises by oxidation heat generated when fuel is added from the fuel adding valve 12 and that added fuel is oxidized at the oxidation catalyst carried on the filter 11. As a result, the PM accumulated on the filter 11 is oxidized and removed. In the filter regeneration control, instead of fuel being added from the fuel adding valve 12, fuel may be supplied to the oxidation catalyst carried on the filter 11 by a secondary fuel injection after a main fuel injection has been performed by the fuel injection valve 10. [0040] Next, the method for estimating the amount of PM remaining during execution of filter regeneration control will be described. When filter regeneration control is performed, it is extremely important to know the PM remaining amount during execution of the filter regeneration control in order to make the end timing of the filter regeneration control, as well as the start timing of the next cycle of filter regeneration control when the filter regeneration control has been stopped while PM still remains on the filter, more appropriate. [0041] Regarding this, the shift in the PM remaining amount during execution of filter regeneration control will be described based on the graph shown in FIG. 2. In FIG. 2, the vertical axis represents the PM remaining amount (i.e., PM accumulation amount) and the horizontal axis represents the time passed since the start of execution of the filter regeneration control. [0042] When the PM accumulated on the filter 11 is oxidized by filter regeneration control, the heat quantity generated by oxidation of the PM increases the greater the PM accumulation amount. Therefore, oxidation of the PM that exists around the oxidized PM is further promoted. During execution of the filter regeneration control, however, oxidation and removal of the PM progresses over time so the PM accumulation amount decreases. As the PM accumulation amount decreases, so does the heat quantity generated by oxidation of the PM, which makes it more difficult for oxidation of the PM existing around the oxidized PM to progress. That is, during execution of the filter regeneration control, the PM accumulation amount gradually decreases over time thus making it more difficult for the oxidation of the PM to progress. As a result, the decrease amount per unit time of the PM remaining amount also decreases over time. Therefore, during execution of the filter regeneration control, the PM remaining amount decreases exponentially over time, as shown in FIG. 2. [0043] Thus, in this exemplary embodiment, the PM remaining amount during execution of filter regeneration control is estimated using a PM removal rate per unit time Rt during the execution of filter regeneration control. The PM removal rate per unit time Rt is the PM removed amount per unit time in units of PM accumulation amount. Unless the operating state of the internal combustion engine changes, this PM removal rate per unit time Rt remains a constant value regardless of the passing of time during the execution of filter regeneration control. [0044] Hereinafter, a specific filter PM remaining amount estimation method during execution of filter regeneration control according to this exemplary embodiment will be described. In this exemplary embodiment, the time elapsing from the start of execution of filter regeneration control starts to be counted upon the start of execution of the filter regeneration control. A PM remaining amount Ql at the point when a specified period of time Δt has been counted from the start of execution of the filter regeneration control is calculated by subtracting the product of the specified period of time Δt, the PM removal rate per unit time Rt, and the PM accumulation amount Qst at the start of regeneration from the PM accumulation amount at the starting point of execution of the filter regeneration control, i.e., from the PM accumulation amount Qst at the start of regeneration. [0045] Then, after the specified period of time Δt has passed from the start of execution of filter regeneration control, and every time that specified period of time Δt is counted, the PM remaining amount at that time is calculated. That is, every time the specified period of time Δt is counted, that specified period of time Δt, the PM removal rate per unit time, Rt and the PM remaining amount at the time the specified period of time Δt at that time started to be counted, i.e., a PM remaining amount Qn. j at the point when counting of the last specified period of time Δt ends, are multiplied together to calculate the PM removal amount to be removed from the filter 11 during the specified period of time Δt at that time. This calculated PM removal amount is then subtracted from the PM remaining amount Qn l at the point when the specified period of time Δt starts to be counted at that time to calculate a PM remaining amount Qn at the point when the specified period of time Δt at that time has been counted. [0046] Here, a PM remaining amount estimation routine for estimating the PM remaining amount during execution of filter regeneration control according to this exemplary embodiment will be described with reference to the flowchart shown in FIG. 3. This routine is stored beforehand in the ECU 20 and is executed every time the crankshaft rotates a specified crank angle while the internal combustion engine 1 is running. [0047] In this routine, first at step SlOl, the ECU 20 determines whether the filter regeneration control is being executed. If this determination is YES, the ECU 20 proceeds on to step S102. If the determination is NO, the ECU 20 ends execution of this cycle of the routine. [0048] In step S 102, the ECU 20 determines whether a specified period of time Δt has been counted since the start of execution of filter regeneration control, i.e., whether a specified period of time Δt has passed since the start of execution of filter regeneration control. As described above, counting of the time that passes from the start of execution of the filter regeneration control is started at the start of execution of filter regeneration control. If the determination in step S102 is YES, the ECU 20 proceeds on to step S103. If, on the other hand, that determination is NO, the ECU 20 proceeds on to step S105. [0049] In step S 103, the ECU 20 calculates a PM remaining amount Ql at the current point, i.e., at the point when the specified period of time Δt has been counted since the start of execution of the filter regeneration control. This calculation is performed by subtracting the product of the specified period of time Δt, the PM removal rate per unit time Rt, and the PM accumulation amount Qst at the start of regeneration from the PM accumulation amount Qst at the start of regeneration. [0050] Next, the ECU 20 proceeds on to step S 104, where it stores the PM remaining amount Ql calculated in step S 103, after which the routine ends. Here, the stored PM remaining amount Ql is the PM remaining amount at the time the next specified period of time Δt starts to be counted (i.e., at the time the second specified period of time Δt starts to be counted). [0051] Meanwhile, at step S 105, the ECU 20 determines whether the time that has passed since the start of execution of the filter regeneration control is longer than the specified period of time Δt. If this determination is YES, then the ECU 20 proceeds on to step S 106. If, on the other hand, the determination is NO, the ECU 20 determines that the time that has passed since the start of execution of the filter regeneration control has not reached the specified period of time Δt and ends execution of this cycle of the routine. [0052] In step S 106, the ECU 20 determines whether the specified period of time Δt since the end of the counting of the last specified period of time Δt has been counted, i.e., whether the specified period of time Δt since the counting of the last specified period of time Δt ended has passed. If this determination is YES, the ECU 20 proceeds on to step S107. If the determination is NO, the ECU determines that the time that has passed since the time the counting of the last specified period of time Δt ended has not reached the specified period of time Δt, and ends execution of this cycle of the routine. [0053] In step S 107, the ECU calculates a PM remaining amount Qn at the current point, i.e., at the point when the counting of the present specified period of time Δt ends. This calculation is performed by subtracting the product of the specified period of time Δt, the PM removal rate per unit time Rt, and the PM remaining amount at the point when counting of the present specified period of time Δt started, i.e., the PM accumulation amount Qn-1 at the point when counting of the last specified period of time Δt ended, from the PM accumulation amount Qn-1 at the point when counting of the last specified period of time Δt ended. [0054] Next, the ECU 20 proceeds to step S 108, where it stores the PM remaining amount Qn calculated in step S 107, after which the routine ends. Here, the stored PM remaining amount Qn is the PM remaining amount at the point when the count of the next specified period of time Δt starts (i.e., at the point when the counting of a n + 1 specified period of time Δt starts, where n is the count of the present specified period of time). [0055] By executing the routine described above, the PM remaining amount every time the specified period of time Δt passes can be more accurately estimated during execution of filter regeneration control. [0056] The specified period of time Δt according to this exemplary embodiment is preferably made as short as possible. Also, when the PM removal rate per unit time Rt is set as the PM removal rate per 1 min, for example, the specified period of time Δt may also be set to 1 min. [0057] Next, the method of setting the PM removal rate per unit time Rt during execution of filter regeneration control according to this exemplary embodiment will be described. The state of PM oxidation and removal during execution of filter regeneration control changes depending on the flow rate and oxygen concentration of the exhaust gas flowing into the filter 11, as well as on the temperature of the filter 11. Therefore, in this exemplary embodiment, the PM removal rate per unit time Rt is changed according to these values. That is, the PM removal rate per unit time Rt is set to a higher value the greater the tendency for PM oxidation and removal to be promoted during execution of filter regeneration control. [0058] More specifically, the heat quantity and oxygen from the exhaust gas supplied for PM oxidation increase the greater the flow rate of exhaust gas flowing into the filter 11. Also, the oxygen from the exhaust gas supplied to the PM increases the higher the oxygen concentration in the exhaust gas. Accordingly, PM oxidation and removal tend to be promoted the greater the flow rate of exhaust gas flowing into the filter 11 and the higher the oxygen concentration in the exhaust gas. For this reason, the PM removal rate per unit time Rt is set to a high value. Similarly, the heat quantity supplied for PM oxidation increases the higher the temperature of the filter 11, which tends to promote PM oxidation and removal, so the PM removal rate per unit time Rt is set to a high value. [0059] By setting the PM removal rate per unit time in this way, it is possible to set the PM removal rate per unit time to a more accurate value, and therefore possible to more accurately estimate the PM remaining amount during execution of filter regeneration control. [0060] The flow rate of exhaust gas flowing into the filter 11 is estimated based on a detection value from the airflow meter 16. Also, the oxygen concentration in the exhaust gas flowing into the filter 11 may either be detected by the oxygen concentration sensor 14, or estimated based on the intake air amount and the fuel injection amount from the fuel injection valve 10. Further, the temperature of the filter 11 is estimated based on a detection value from the exhaust gas temperature sensor 13. [0061] Next, a second exemplary embodiment of the invention will be described. The general structure of an internal combustion engine and the intake and exhaust systems thereof according to the second exemplary embodiment is similar to that described in the first exemplary embodiment above, so a description thereof will be omitted. [0062] Here, a method for regenerating a filter when the operating state of the internal combustion engine 1 has changed during filter regeneration control according to the second exemplary embodiment will be described. [0063] When the operating state of the internal combustion engine 1 changes, that change may produce changes in the flow rate and oxygen concentration of the exhaust gas flowing into the filter 11, as well as in the temperature of the filter 11. Therefore, if the operating state of the internal combustion engine 1 changes during execution of the filter regeneration control, so too might the PM removal rate per unit time Rt. [0064] If the PM removal rate per unit time Rt drops as a result of a change in the operating state of the internal combustion engine 1 during execution of filter regeneration control, the duration of the filter regeneration control necessary to oxidize and remove the remaining PM increases. If the filter regeneration control is executed for too long, however, it may lead to a deterioration in fuel efficiency and exhaust gas emissions. [0065] Therefore, in this exemplary embodiment, by executing the control routine shown in FIG. 4, the filter regeneration control is stopped when the operating state of the internal combustion engine 1 changes during execution of the filter regeneration control and it has been determined that the duration of the filter regeneration control is too long. [0066] FIG. 4 is a flowchart illustrating a routine for controlling the stopping or continuation of the filter regeneration control according to this exemplary embodiment. This routine is stored in advance in the ECU 20 and is executed every time the crankshaft rotates a specified crank angle while the internal combustion engine 1 is running. [0067] In this routine, first in step S201, the ECU 20 determines whether the filter regeneration control is being executed. If this determination is YES, the ECU 20 proceeds on to step S202. If, on the other hand, that determination is NO, the ECU 20 ends execution of this cycle of the routine. [0068] In step S202, the ECU 20 determines whether the operating state of the internal combustion engine 1 has changed. If this determination is YES, the ECU 20 proceeds on to step S203. If that determination is NO, the ECU 20 proceeds on to step S207, where the filter regeneration control is continued. [0069] In step S203, the ECU 20 changes the value of the PM removal rate per unit time Rt based on the temperature of the filter 11 and the flow rate and oxygen concentration of the exhaust gas after the change in operating state of the internal combustion engine 1. Because the temperature of the filter 11 and the flow rate and oxygen concentration of the exhaust gas become values in accordance with the operating state of the internal combustion engine 1, the relationship between the operating state of the internal combustion engine 1 and the PM removal rate per unit time Rt may be obtained through experimentation or the like and mapped beforehand, and the value of the PM removal rate per unit time Rt may be derived from this map. [0070] Next, the ECU 20 proceeds on to step S204, where it calculates the duration of the filter regeneration control necessary to oxidize and remove the remaining PM based on the PM removal rate per unit time Rt that was changed in step S203 and a PM remaining amount Qch at the present time, i.e., at the time when the operating state of the internal combustion engine 1 changed. The PM remaining amount Qch at the present time is estimated by the method for estimating the remaining amount of PM in the filter during execution of the filter regeneration control according to the first exemplary embodiment. [0071] Next, the ECU 20 proceeds on to step S205, where it is determined whether the duration of the filter regeneration control calculated in step S204 is equal to, or greater than, a specified duration Δtc. Here, the specified duration Δtc is the time of a threshold value at which it is possible to determine that there is a possibility of fuel efficiency and exhaust gas emissions deteriorating due to the filter regeneration control being executed for too long. If the determination in step S205 is YES, the ECU 20 proceeds on to step S206. If the determination in step S205 is NO, on the other hand, the ECU 20 proceeds on to step S207. [0072] In step S206, the ECU 20 stops the fuel regeneration control and ends execution of this cycle of the routine. [0073] By executing the routine described above, the filter regeneration control can be inhibited from being executed for too long. As a result, it is possible to suppress deterioration in fuel efficiency and exhaust gas emissions.